The present invention relates to method and apparatus for control of positioning, and more particularly to method and apparatus for positioning control of a head in an optical disk device or a magnetic disk device.
In a magnetic disk device, with the aim of recording information at a predetermined position on the recording surface of a recording medium or reading information already recorded, positioning control is carried out to move and position a magnetic head to a predetermined track. In this case, it is general practice to conduct positioning control by performing switching between a plurality of control systems in accordance with the difference between the present position of the head standing for a controlled object, and a target position (target track).
Namely, when the difference between the present position and the target position exceeds a predetermined value, control is performed by using a velocity control system on the basis of a target velocity corresponding to the difference. As the head approaches the target position, positioning control is performed by using a position control system on the basis of the difference between the present position and the target position. The above method is effective to achieve fast and accurate positioning to the target position.
Thus, switching is effected from the velocity control system to the position control system, and if initialization in the position control system is improper, transient characteristics during switching between the control systems cause a response waveform of the head to oscillate or to be delayed, and much time is needed for positioning to the predetermined position.
In the past, some methods to solve these problems have been proposed. For example, JP-A-56-114176 discloses a method of improving response time after switching from the velocity control system to the position control system. In this proposal, a voltage proportional to a motor speed immediately before switching is stored in an integrator included in the position control system.
JP-A-1-138666 discloses a method to minimize transient response immediately after switching to the position mode. In this proposal, capacitors of integrators included in the position control system are charged with transient response correction values for individual tracks. These values are used as initial values of the integrators immediately after switching to the position control system.
In the above prior art, an the integrator is provided in the position control system, and an initial value is set in the integrator to ensure smooth and fast response upon switching.
Also, in the prior art, setting of an optimal initial value can be realized under the condition that characteristics of the movable part, including the head standing for the controlled object, are known, and besides the characteristics do not change when the movable part is moved to another position.
Practically, however, the force constant of the motor adapted to move the head and the gain of the power amplifier are not constant. For example, in a voice coil motor frequently used as an actuator for moving the head of magnetic disk device, the force constant of the motor is decreased under the influence of leakage of magnetic flux at the motor end.
In the case of a magnetic disk device of the so-called sector servo scheme wherein servo data is recorded in the form of a sector on the disk surface and positioning of the head is carried out by reading the data, position sensitivity may differ for the inner zone and the outer zone of the disk depending on the manners of writing the servo signals. Thus, in general, characteristics of the controlled object change, and the loop gain of the control system changes. Accordingly, in the initial value setting for the integrator according to the conventional method, coefficients in a computation equation used for initial value computation deviate from actual values, and as a result, the determined initial values may sometimes not be optimal values, making it difficult to achieve smooth switching from the velocity control system to the position control system.
In addition, the mechanical system of the controlled object has a plurality of natural frequency modes, and when the results of the theoretical calculation without taking the natural frequency modes into consideration are applied directly, satisfactory response sometimes cannot be obtained.